Excessive exercise can be as addictive as heroin, according to US scientists, and stopping can lead to withdrawal symptoms. The scientists believe that extreme exercise causes increases in endogenous opioid peptides (endorphins), which act in the brain a manner similar to chronic administration of opiate drugs. The psychologists observed a group of rats, some of which exercised excessively on a wheel and others which were inactive. When they administered naloxone (an opioid receptor antagonist that blocks the actions of opioids) to the animals, active and inactive rats responded very differently. The active rats showed withdrawal symptoms similar to those observed in rats that were addicted to heroin. The non-active rats were unaffected. (Running and addiction: precipitated withdrawal in a rat model of activity-based anorexia. Behav Neurosci. 2009 Aug;123(4):905-12).